Hot melt adhesives are generally a solid thermoplastic material which quickly melts upon heating and then sets to a firm bond on cooling. In contrast to most other types of adhesives which typically set by evaporation of a solvent or emulsion medium, hot melt adhesives are generally homogenous mixtures. In the manufacture of hot melt adhesives, a thermoplastic material is typically compounded (e.g., melt blended) with one or more tackifiers, waxes, antioxidants and/or other stabilizing additives, and then cooled to form rods, pellets, chicklets, bricks or the like for sale to an end user. In the use of hot melt adhesives, the rods, pellets, chicklets, bricks or the like are typically heated in a reservoir at about 120.degree.-250.degree. C. (and often kept at this temperature in the reservoir for 24 hours or more) from which it is supplied to a substrate by pumping or pressure extrusion through nozzles or slot dies. In contrast to the solvent or emulsion type adhesive which requires time for the evaporation of the solvent or emulsion medium, an almost instantaneous bonding can be obtained with hot melt adhesives.
The use of thermoplastic ethylene polymers in hot melt adhesive compositions is well known. For example, in U.S. Pat. No. 3,485,783 there is described a hot melt adhesive composition containing at least 40% by weight of a polymerized rosin ester tackifier and at least 25% of a copolymer of ethylene and acrylic acid or alkyl acrylate. However, in such adhesive compositions it has heretofore been necessary to limit the acrylic acid content of the copolymers to twelve percent by weight or less because of compatibility problems with waxes and tackifiers. The immiscibility of ethylene-acrylic acid copolymers containing more than 5 mole percent acrylic acid has also been reported in adhesive compositions including a wood rosin tackifier and a petroleum wax in U.S. Pat. No. 3,869,416. Similarly, U.S. Pat. No. 4,284,541 reported unsuitable fluidity in a pressure sensitive adhesive composition of a tackifier and a modified ethylene-alpha-olefin copolymer to which had been grafted more than ten percent by weight of the copolymer of an unsaturated carboxylic acid.
Hot melt adhesive compositions containing a tackifier and an ethylene-unsaturated carboxylic acid copolymer in which the acrylic acid content is greater than twelve percent have heretofore required the addition of other ingredients, including oil as in U.S. Pat. No. 4,283,317, or an alkenyl succinic anhydride as in U.S. Pat. No. 4,325,853. In many applications, oils and waxes are undesirable because of reduced physical properties, chemical resistance and/or adhesion.